Downloads WebUI: Remove unused i18n.html.

This file was renamed at https://codereview.chromium.org/1409933012, but somehow
both the new and previous files remained in the repository.

Bug: None
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I2ea7152f168bbae9290b961397d70f4f853ed04b
Reviewed-on: https://chromium-review.googlesource.com/990727Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547584}

chrome/browser/resources/md_downloads/i18n.html

-<script src="chrome://resources/js/load_time_data.js"></script>
-<script src="chrome://downloads/strings.js"></script>